Implantable medical devices for producing a therapeutic result in a patient are well known. Examples of such implantable medical devices include, but are not limited to, implantable drug infusion pumps, implantable neurostimulators, implantable cardioverters, implantable cardiac pacemakers, implantable defibrillators and cochlear implants. Of course, it is recognized that other implantable medical devices are envisioned which utilize energy delivered or transferred from an external device.
A common element in many of these implantable medical devices is the need for electrical power in the implanted medical device. The implanted medical device may require electrical power to perform its therapeutic function whether it be driving an electrical infusion pump, providing an electrical neurostimulation pulse or providing an electrical cardiac stimulation pulse. This electrical power is derived from a power source.
Typically, a power source for an implantable medical device is a rechargeable power source. In this form, an internal power source, such as a battery, can be used for direct electrical power to the implanted medical device. When the battery has expended, or nearly expended, its capacity, the battery can be recharged transcutaneously, via inductive coupling from an external power source temporarily positioned on the surface of the skin.
The external power source typically may have an external antenna that is placed in the proximity of a corresponding internal antenna associated with the implantable medical device.
In order to charge or recharge the implantable medical device's rechargeable power source, it is typical for the user to place an external charger, or an antenna associated with an external charger, in the proximity of the implantable medical device, or in the proximity of an internal or secondary antenna or coil associated with the implantable medical device. Optimally, the primary coil of the external charger will be aligned as closely as possible with the secondary coil of the implantable medical device minimizing the distance between the two coils and providing a relatively efficient transfer of energy between the external charger and implantable medical device.